Most climate scientists agree that Earths warming has accelerated, creating
debate about the extent to which humans have driven that change. While people
have been putting their instruments to work, layers of fossilized marine creatures,
resting undisturbed offshore of Southern California, have acted as an independent
record of ocean temperature for millennia. Now, data from such layers is mirroring
the same warming trend that instruments have shown  suggesting humans
are contributing to global warming.

David Field, an oceanographer at the Monterey Bay Aquarium Research Institute
in Moss Landing, Calif., and his colleagues studied species of plankton known
as foraminifera, or forams, which are highly sensitive to temperature changes
in the ocean. Various species of the microscopic creatures inhabit oceans around
the world, and when they die, their calcite shells collect on the seafloor.

The
team was not being imprudent by relating their findings to human-caused
warming.

Peter
deMenocal,
Lamont-Doherty Earth Observatory

Looking at different layers of ocean sediments containing shells, Fields
team found that warm-water tropical species took hold in oceans off Southern
California during the last century in levels unseen throughout the entire 1,400-year
span of the core sample. The entire sample spanned centuries-long events such
as the Little Ice Age and the Medieval warm period, but variability on the scale
of decades was more dominant in the cores, Field says, and the last
several decades stand out as anomalous.

The most compelling part of the study, which was published in the
Jan. 6 Science, is that unlike previous studies of ocean temperatures,
Fields teams research puts the modern world within the context
of the world before people were really present in large numbers, says
Peter deMenocal, a paleooceanographer at Lamont-Doherty Earth Observatory in
New York. And, he says, the study shows that whatever has happened in
the last century is really unique within the context of the last 15 centuries.

Field and colleagues studied 1- to 3-meter-long cores from the Santa Barbara
Basin that contained annual layers of sediments and forams. In the basin, low
oxygen levels have kept animals from venturing to the seafloor and disturbing
the layers, allowing for well-preserved annual layers of sediments with forams,
similar to the rings on a tree.

Field counted species within two-year intervals for about the last 260 years
and then species within five-year intervals back about 1,400 years. The counts,
he says, revealed that tropical and subtropical species increased in abundance
after 1925 and then dramatically increased in the mid-1970s. Abundances of different
foram species that prefer cool water dropped during those same times.

Known natural variability in climate on the scale of decades and centuries does
not account for the sudden changes observed in the core measurements, Field
says. The best explanation, he says, is that greenhouse gas emissions
are affecting marine ecosystems.

If the study stood alone, without support from other studies showing the same
trend, then one might be a little bit skeptical, deMenocal says.
He notes that changes in nutrients, runoff from development or other non-greenhouse-gas-related
human influences could affect the ecosystem and not be tied to global warming.
The timing and nature of the transition, however, deMenocal says, are consistent
with a number of other studies, most of which were focused on land.

This is the ocean part of that story, deMenocal says. Field and
colleagues make a good case that post-Industrial Revolution climate changes
are anomalous within the context of a longer period, deMenocal says, and the
team was not being imprudent by relating their findings to human-caused
warming.